Miscible displacement of petroleum using sequential addition of carbon disulfide and a hydrocarbon solvent

ABSTRACT

A miscible displacement process for the recovery of petroleum from a petroleum bearing formation is performed in situ by the use of a solvent system miscible with the petroleum, the solvent system comprising sequential slugs of carbon disulfide and a hydrocarbon solvent.

lwe g United States Patent [191 Allen et al.

[ 51 Nov. 12, 1974 MISCIBLE DISPLACEMENT OF PETROLEUM USING SEQUENTIALADDITION OF CARBON DISULFIDE AND A HYDROCARBON SOLVENT [75] Inventors:Joseph C. Allen, Bellaire; Jack F.

Tate; Roland B. Stelzer, both of Houston, all of Tex.

[73] Assignec: Texaco Inc., New York, NY.

[22] Filed: May 4, 1973 [2]] App]. No.: 357,407

52 us. C1. 166/273 [51] Int. Cl E21b 43/16 [58] Field of Search166/266-275, 166/305 [56] References Cited UNITED STATES PATENTS2,725,106 11/1955 Sperow 166/268 3,047,063 7/1962 Connully, .lr. ct al.166/273 3,131,760 5/1964 Arendt et a1 166/268 3,249,157 5/1966 Brighamet a1 166/273 3,729,053 4/1973 Froning 166/273 Primary Examiner-James A.Leppink Attorney, Agent, or Firm-Thomus H. Whuley; C: G. Ries 5 7ABSTRACT A miscible displacement process for the recovery of petroleumfrom a petroleum bearing formation is performed in situ by the use of asolvent system miscible with the petroleum. the solvent systemcomprising sequential slugs of carbon disulfide and a hydrocarbonsolvent.

3 Claims, 1 Drawing Figure MISCIBLE DISPLACEMENT OF PETROLEUM USINGSEQUENTIAL ADDITION OF CARBON DISULFIDE AND A HYDROCARBON SOLVENTEACKGROUND OF THE INVENTION l. Field of the Invention This inventionrelates to a process for recovering petroleum by miscible displacement.

2. Description of the Prior Art Various methods for inducing therecovery of petroleum from underground reservoirs are in existence.These methods include injecting water, steam or some aqueous basedmixture to drive the oil from the reservoir. These displacementprocesses are inefficient. The inefficiency of these displacementprocesses is partly due to the retentive forces of capillarity andinterfacial tension. Miscible flooding provides a method for efficientlydisplacing the petroleum from a reservoir.

In, miscible flooding, solvent for the petroleum is introduced into thereservoir and driven through the reservoir. Dissolution of the petroleumby the solvent permits no two phase system between the solvent and thepetroleum to exist atlthe conditions of temperature and pressureexisting in the reservoir. Therefore, the retentive forces ofcapillarity and interfacial tension are nonexistent. These forcesdecrease the displacement efficiency of a recovery process where thedriving fluid or displacing agent and the petroleum exist as two phasesin the reservoir.

In a miscible flood process the solvent has the capability of mixingcompletely with the petroleum in the reservoir. A transition zone isformed at the leading edge of the solvent between the solvent and thepetroleum in which miscibility exists between the solvent and thepetroleum. For economic reasons the solvent is normally injected as aslug followed by another fluid such as a gas or an aqueous fluid todrive the solvent slug and the petroleum through the reservoir.

In displacement processes in general, the ideal sought after ispiston-like displacement. That is, the displacing fluids should ideallypresent a flat front to the petroleum in the reservoir and displace ituniformly through the reservoir. Most miscible solvent slugs arefollowed by. an aqueous fluid to drive them through the reservoir.Moreover, most miscible solvents have heretofore been light hydrocarbonswith densities less than water. Problems have arisen with suchprocesses, however.

In a vertical miscible flood, for example, using a light hydrocarbonsolvent slug followed by water, the water will tend to finger throughthe less dense solvent, destroying piston-like displacement andresulting in premature breakthrough of the displacing medium water.Further, there are certain pertroleum deposits which are only partiallysoluble in the prior art solvents. One type of petroleum which is onlypartially soluble in prior art solvents is the tar sand oils.

Throughout the world there are various known locations wherein the earthcontains large deposits of tar sands. For example, one of the mostextensive and best known deposits of this type occurs in the Athabascadistrict of Alberta, Canada. In the tar sands in such deposits, the oiltypically has a density approaching or even greater than that of water.The Athabasca tar sands extend for many miles and occur in varyingthicknesses of up to more than 200 feet. Although in some places theAthabasca tar sands are disposed practically on the surface of theearth, generally they are located under an overburden which ranges inthickness from a few feet to as much as 1,000 or more feet in depth. Thetar sands located at these depths constitute one of the worlds largestpresently known petroleum deposits. In these sands, the oil contentranges between about l0 and 20 percent by weight, although sands withlesser or greater amounts of oil content are not unusual. Additionally,the sands generally contain small amounts of water in the range of fromabout 1 to 10 percent by weight.

The oil present in and recoverable from Athabasca tar sands is usually arather viscous material ranging in specific gravity from slightly below1.00 to about 1.04 or somewhat greater. At a typical reservoirtemperature, e.g., about 48 F., this oil is immobile, having a viscosityexceeding several thousand centipoises. At higher temperatures, such astemperatures above about 200 F. this oil becomes mobile, withviscositites of less than about 343 centipoises, and the tar sands areincompetent. Since this tarry material does not generally command a veryhigh price, particularly particularly when in its crude state, itsseparation and recovery must involve a minimum of expenditure in orderto be economically attractive for commercial practice.

The bitumen in the tar sands is a petroleum material which is notcompletely soluble in most prior art solvents such as LPG, mixtures oflight hydrocarbons having from two to six carbon atoms or propane.

In copending applications Ser. No. 357,409 filed May 4, 1973 and Ser.No. 357,414 filed May 4, 1973, processes were disclosed which used asolvent miscible with the said oils. One of those solvents was carbondisulfide. However, carbon disulfide has drawbacks which may limit itsexclusive use in some situations. For example, carbon disulfide ishighly toxic and very flammable. Carbon disulfide is also expensive atthe present time. This invention will alleviate many of these problemswhile retaining much of the unique beneficial properties of a carbondisulfidemiscible flood.

SUMMARY OF THE INVENTION The invention is a process for the recovery ofpetroleum from a reservoir by miscible displacement. The processinvolves introducing sequential slugs of different solvents capable ofdissolving fonnation hydrocarbons, one of the solvent slugs having adensity greater than water, into the upper portion of a reservoir andforcing the sequentially added solvent slugs through the reservoir tosome point lower than the point of introduction by means of an aqueousfluid and/or gas and producing the solvent and extracted fluids from thereservoir at this point. The solvents in one slug comprising carbondisulfide or chlorinated hydrocarbons and the solvent in another slugcomprising aromatic hydrocarbons or aliphatic hydrocarbons.

BREIF DESCRIPTION OF THE DRAWING The drawing depicts the verticalsolvent drive process of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention is animprovement in the displacement of heavy oil, such as tar sand oil. Theimprovement comprises a first downward displacement with a sequence orpetroleum solvents, at least one having a density greater than thedriving fluid and preferably a viscosity less than the driving fluid,followed secondly by displacing the solvent-petroleum solution alsodownwardly with a driving fluid such as water or gas.

Although aqueous fluids such as water are the preferred fluids fordisplacing the solvent slugs through the reservoir, any fluid havingfavorable properties may be used. The displacing (driving) fluid musthave a density equal to or less than the solvent in at least one of thesequential slugs and preferably more viscous. Although not absolutelynecessary, the fluid should also be substantially unreactive with thesolvents.

Two types of solvent are necessary for the operation of this invention.One solvent must be heavier than water. Examples of specific solventsheavier than water include but are not limited to carbon disulfide andchlorinated hydrocarbons such as methylene dichloride and carbontetrachloride.

The other type of solvent is a hydrocarbon. The hydrocarbon may bearomatic or aliphatic or a blend of each. Benzene, toluene and xyleneare typical examples of aromatic hydrocarbons useful in the process ofour invention. Aliphatic solvents useful in our invention include lighthydrocarbons from two to six carbon atoms. Debutanizer bottoms is asolvent which is an example of blended aromatic and aliphatichydrocarbons. This brief enumeration is not intended to be exhaustive.The hydrocarbons useful in our invention are far too numerous to mentionand will be obvious to those skilled in the art in light of theteachings contained herein.

It is a peculiar property of tar sand oil that only certain componentsof it are soluble in aliphatic or aromatic hydrocarbon solvents.Consequently, the use of these solvents alone in an oil recovery processfrom a tar sand deposit will leave many valuable hydrocarbons behind.

Most tar sand oils or bitumens are essentially completely soluble incarbon disulfide. Also, carbon disulfide has a density greater thanwater. Thus, carbon disulfide sequentially added with an aromatic oraliphatic solvent will solve the above problem. The solvent slugs willdissolve all components of the tar sand oil.

In certain applications carbon disulfide is preferred over the otherheavier than water solvents because of its unique properties or ease ofmanufacture and recovery. Where the recovered crude is to becatalytically treated in a refinery, for example, carbon disulfide ispreferred. It is a characteristic of covalently bonded halogens such asthose found in halogenated hydrocarbons that they tend to poison somerefinery catalysts. Carbon disulfide does not and in addition is quiteeasily removed from recovered crude by physical separation processes tobe reused again, leaving the crude substantially free of carbondisulfide. Carbon disulfide may also have a great economic advantageover halogenated hydrocarbons since it may be manufactured by thereaction between coke (carbon) and sulfur. Coke and sulfur are oftenfound in excess near prolific tar sand deposits such as the Athabascatar sands of Canada. The use of these materials would be an aid toconservation of the environment.

The solvent slugs may be injected into the formation in any order andthe sequences may be repeated if desired. However, it is especiallypreferred that the hydrocarbon slug precede the slug of solvent heavierthan water. Both are then followed by a driving fluid. This sequencewill allow the ligher hydrocarbon slug to be beneath the heavier thanwatersolvent slugs and will prevent any possibility of viscous fingeringof the driving fluid through the slugs. That is, in a downward drive thelighter driving fluid will ride above the carbon disulfide or otherheavier than water solvent slug providing piston-like displacement ofthe solvent slugs.

The process of our invention may be carried out by a variety oftechniques. In one technique, for example, at least two wells areneeded, one for injection and another for production. The solvent slugsmay proceed through the formation horizontally from the injection wellto the production well, but the benefits of this invention becomegreater as the angle the solvent proceeds through the formationapproaches from the horizontal. Ideally, the interface between thesolvents and aqueous fluid driving it should be horizontal. Thisconfiguration allows gravity stabilization to have its maximum effect onthe system.

The process of this invention is operable in a variety of petroleumreservoirs containing petroleum of widely diff riuasratit s O p er ed mod ever, is to recover tar sand oil using a solvent sequence of carbondisulfide and/or chlorinated hydrocarbons and an aromatic and/oraliphatic hydrocarbon. As pointed out previously, it is a characteristicof the bitumen constituents of tar sand oil or petroleum that they aresoluble in carbon disulfide and less soluble in most other solvents.

A very important advantage of using carbon disulfide is the lack of anemulsification of the separate driving water and carbon disulfidephases. The phases separate into distinct layers easily separable fromeach other. This feature is advantageous for many reasons. For example,emulsification within the formation could lead to a reduction inpermeability due to what is commonly known as emulsion blockage. Thelack of emulsification when carbon disulfide is used prevents thisproblem from occurring. Also, emulsification could destroy piston-likedisplacement. Another advantage of the lack of emulsion forming tendencybetween carbon disulfide and water occurs when the solvents, bitumen andwater are produced and separation of the carbon disulfide is desired.Emulsion formation would distinctly hamper these operations.

The size of solvent slugs to be used will depend on the solvent chosenand the degree of recovery desired. The degree of recovery desired is amatter of economics and may be determined by those skilled in the artwithout engaging in inventive effort. As an aid in determining the sizeof slugs needed the following procedure may be used but is not intendedto limit the scope of our invention or tie it to any routine calculationprocedure. The size of a slug of solvent, for example, may be calculatedby a formula such as:

Solubility of bitumen in solvent X amount of bitumen per acre-foot offormation X acre-feet in formation X degree of depletion desired(decimal) amount of solvent required Routine laboratory experimentationmay be used to determine the solubility of a given bitumen in aparticular solvent and core analysis will yield information on theamount of bitumen per acre foot of formation. Thus,

the size of solvent slug for any field may be determined.

The temperature of the solvent slugs should be low enough to avoidhaving the carbon disulfide react with water. Normally the slugs will beintroduced at ambient temperature and will take on the reservoirtemperature in a short period of time. In Canadian tar sands, forexample, the reservoir temperature is about 45 F. In no case when carbondisulfide is used should its temperature be above that at which carbondisulfide reacts with water, about 400 F.

The temperature of the displacing or driving water should not be so lowthat, in combination with dissolved salt content, its density exceedsthat of the carbon disulfide or other heavy solvent.

A fairly thick reservoir is preferred in the process of this inventionto allow as near to a verical miscible flood as possible. The placementof the injection and production wells is related. They should besituated so that the injection of the solvents and an aqueous drivefluid takes place at a point in the reservoir above the point where theproduction is taken from the reservoir. The lateral as well as verticalspacing of the production and injection points should be such that ablanket of solvent followed by water will cover the largest area of thereservoir consistent with economics. The above factors should beunderstood as given to explain how to maximize the effectiveness of thisinvention. However, the invention should not be construed as limited toany particular well configuration or reservoir type.

MINING TECHNIQUES In one embodiment of the invention a dump flood may beperformed. This involves drilling a large diameter hole into the crestalportion of a tar sand formation, for example. A sequence of solvents asdescribed heretofore, is dumped into the cavity. The solvents willgravitate into the formation and be imbibed by the formation displacingthe oil toward producing well completed lower in the formation. Anaqueous fluid such as water is introduced into the cavity to maintain alayer over the solvents. This prevents evaporation of the solvents. Dumpflooding is particularly useful where the formation is near to thesurface of the earth where high injection pressures could result inbreakthrough of injected fluids through the overburden to the surface.Near to the surface could include, for example, depths of 200 to 300feet or more.

A similar mining technique could also be used where tar sands outcrop atthe surface. A dam of earth, for example, could be constructedsurrounding the outcrop providing a recepticle for the injection ofsolvent and water..Production wells drilled down dip from the outcropwould withdraw the combined solvent-tar extract.

As a rule of thumb so-called mining techniques are normally'consideredto be feasible where the ratio of the distance from the surface of theearth to the thickness of the tar sand reservoir or pay is one or less.

lN SITU RECOVERY TECHNIQUES For deeper formations other embodiments ofour invention involve having at least one production well and oneinjection well. The preferred configuration is that the point ofinjection be far enough above the point of production to allow asomewhat vertical traverse for fluids entering the reservoir through theinjection well and being produced through the production well. The slugsof solvents heretofore described are introduced through the injectionwell followed by an aqueous fluid to push the solvents through thereservoir to the production well.

A variation of this includes using one well having upper and lowerperforation and which is internally equipped so as to avoid fluidcommunication in the well between the upper and lower perforations. Thesolvent slugs are injected through the upper perforations and producedthrough the lower perforations along with petroleum driven ahead of thesolvents. If desired, the solvents may be followed by an aqueous fluid.

The process of our invention may be illustrated by reference to theaccompanying figure which depicts one embodiment of our invention. Otherembodiments will, of course, occur to those having had the benefit ofthe teachings contained herein.

A reservoir containing a very viscous petroleum 10 is penetrated by aninjection well 11 and production wells 12 and 13, the injection wellhaving communication with the reservoir through perforations 14 abovethe perforations 15 in the production wells. The figure represents apoint in time well into the recovery program where sequential slugs ofsolvents, toluene l6 and carbon disulfide 17 have been injected into thereservoir through the injection well followed by an aqueous fluid 18which is presently being injected into the injection well. The petroleum19 is moving towards the projection well perforations where it isproduced. The interfaces shown between the solvent slugs and the tolueneand the petroleum are, of course, not as distinct as shown in thisillustration.

EXPERIMENTAL The superior performance of carbon disulfide followed bywater in removing tar sand oil has been demonstrated in laboratoryexperiments. A weighed quantity of tar sand was placed in a glass tube1.5 inches in diameter above a glass wool filter to retain the sand.Sufficient carbon disulfide was introduced into the tube to completelysaturate and cover the tar sand. Water was then placed above the carbondisulfide-tar sand layer. The water formed a distinct layer of waterformed above the carbon disulfide-tar sand system. The assembly wasclosed and shut-in over night. The next day a stopcock at the bottom ofthe assembly was opened and all fluid allowed to drain out into agraduate cylinder. A two phase system formed in the graduate cylinder: alower phase consisting of carbon disulfide and dissolved tar sand oil,and an upper clear water phase. The water passed easily through the tarsand and was drawn off easily since the phase boundary was dis- I tinct.No emulsion formation was noted at the interface between the water andthe carbon disulfide-tar sand oil mixture.

Virtually all of the carbon disulfide was recovered from the originaltube. The carbon disulfidetar sand oil mixture was vacuum-distilled. Nofrothing or foaming occurred during the operation even though thesolution actually boiled. All of the carbon disulfide was removed andonly pure, heavy, viscous tar sand oil remained.

We claim:

I. A process for recovering petroleum from a reservoir comprising:

7 8 sequentially introducing at least two solvent slugs the point ofintroduction of the solvent slugs.

into the reservoir, 21 first solvent slug having a den- 2. A process asin claim 1 wherein the reservoir is a sity greater than water and asecond solvent slug tar sand reservoir. having a density less thanwater, followed by an 3. A process as in claim 2 wherein the firstsolvent aqueous driving fluid wherein the second solvent 5 slugcomprises carbon disulfide and the second solvent slug is injected priorto the first solvent slug, and slug comprises a hydrocarbon. producingpetroleum and solvents at a point below

1. A PROCESS FOR RECOVERING PETROLEUM FROM A RESERVOIR COMPRISING:SEQUENTIALLY INTRODUCING AT LEAST TWO SOLVENT SLUGS INTO THE RESERVOIR,A FIRST SOLVENT SLUG HAVING A DENSITY GREATER THAN WATER AND A SECONDSOLVENT SLUG HAVING A DENSITY LESS THAN WATER, FOLLOWED BY AN AQUEOUSDRIVING FLUID WHEREIN THE
 2. A process as in claim 1 wherein thereservoir is a tar sand reservoir.
 3. A process as in claim 2 whereinthe first solvent slug comprises carbon disulfide and the second solventslug comprises a hydrocarbon.